In the manufacture of web based products, such as paper, textiles and certain plastics, a web of material is moved along a serpentine path through various stations wherein a different manufacturing operation is performed on the web at each station. The total length of a web moving through such a path can measure several hundred feet and can measure several feet in width. Should the web break during the manufacturing process, significant downtime can occur while the web is rethreaded through the different stations. As will be appreciated, such downtime can result in substantial cost to the manufacturer. An additional consequence of a web break is the detrimental effect on product quality if breaks are occurring too frequently.
Therefore, a need exists in the manufacture of web based products for methods and apparatus for preventing web breaks. One aspect of the invention described and claimed herein is the prevention of such breaks through the investigation of web flutter. Flutter is that phenomenon where the web moves in a direction substantially perpendicular to the direction of travel, which movement has one or more amplitudes and frequencies. Since touching the web during production is to be avoided, if possible, it will be important to determine web flutter in a fashion which does not contact the web.
Devices have been previously disclosed for the determination of web flutter in a non-contact fashion. U.S. Pat. No. 4,496,428 -- Wells and related U.S. Pat. No. 4,501,642 -- Wells discuss the use of reflected light in order to determine the amplitude and frequency of web flutter during paper manufacture so that tension in the paper web can be maintained at some desired level. Basically, it appears that light is projected from a source, mounted adjacent the paper path, onto a moving web. The source is mounted so that the reflection is directed onto an optoelectronic detector. Frequency is said to be determined from the time it takes the reflected light to scan back and forth across the detector. Amplitude is determined from the magnitude of the reflected light. Although this patent suggests the use of radar or ultrasonic devices for determining flutter, no method or apparatus is disclosed.
U.S. Pat. No. 4,637,727 -- Ahola et al. also discusses the use of light to make a non-contact determination of web flutter. It is said in that patent that the minimization of flutter results in the probability of a web break being smaller. Basically, it appears that flutter amplitude and frequency are determined through the use of a high frequency distance measuring scheme. A light pulse is reflected off a moving paper web and directed onto a photodiode. The time it takes the light to travel from its source to the photodiode is measured. Over a period of time, sufficient measurements can be made to determine the frequency and amplitude of web flutter. This patent also suggests the use of capacitance or ultrasound to determine web flutter. However, for different reasons, each of these techniques is rejected in favor of the light-based technique.
The problem with these previously described devices is that they do not appear to be pracatical in the manufacturing environment. For example, in the manufacture of paper it will be necessary to determine flutter within web pockets where temperatures can reach 180.degree. F. or higher. Also, if a web break occurs in or around the region where flutter is being determined the device being used can be struck either by the advancing web or by the end of the web, i.e., the break tail. The forces involved in such contacts can be significant enough to damage light-based devices. Moreover, and perhaps more significant, is the fact that since the prior devices were based on wave reflection from the surface of the web, the stationary sensors of such prior devices were not capable of reliable detection. This key deficiency is the result of the incidental web surface not being normal to the sensor as will occur as a result of flutter.
It is also a practice in web manufacturing that if flutter appears to be too severe such that a web break or excessive wrinkling is feared, the flutter is reduced by slowing the movement of the web through the machinery. As will be appreciated, the slowing of the web results in the manufacturing equipment being operated at less than capacity, which is economically undesirable.
Consequently, a need still exists for a method and apparatus to determine web flutter in a manufacturing environment, which method will remain reliable even in the presence of varying amount of flutter.